1. Field of the Invention
The present invention relates to an air intake system of an internal combustion engine and, more particularly, to an air intake system which is adapted to vary the effective length thereof in accordance with engine operating conditions.
2. Description of the Related Art
It is well known in the design of an air intake system of an internal combustion engine to cause the resonance frequency of an air column flowing through an air intake system to match the operating frequency of intake valves. The intake air is effectively charged into engine cylinders due to the air column flowing under the effect of inertia, thereby enabling a high output torque of the engine. This type of intake air charging is known as supercharging by inertia.
The resonance frequency of an air column in the intake system is determined by the effective length of the intake system. Usually, the effective length of intake system is constant. In automotive engines, however, the speed of revolution of the engine varies to a considerable extent, so that the speed range is limited in which inertia supercharging can be utilized. If the effective length of an air intake system is selected to be longer for the purposes of obtaining the advantage of inertia supercharging and, hence, a higher output torque at a lower engine speed, then that effective length would be too large for the engine to be supercharged by inertia at a higher engine speed. Conversely, if the effective length of the intake system is matched to a higher engine speed, in order to increase the maximum output of the engine, then it is not possible to improve the output torque at a lower speed.
To overcome this discrepancy, there has been proposed in the prior art an air intake system for use with a 4-valve engine wherein two intake tubes are provided for each cylinder, one of the intake tubes being matched to a higher engine speed and the other to a lower speed (Japanese Unexamined patent publication No. 57-110765). This intake system fails to improve the engine output to a required level because one of the two intake tubes necessarily has a length which is not matched with an actual engine speed.
There have also been proposed intake systems wherein the equivalent effective length of the system is rendered variable in accordance with the rotational speed of the engine. For example, Japanese Unexamined patent publication No. 56-115818, Japanese Unexamined Utility Model publication No. 57-92021, Japanese Unexamined Utility Model publication No. 57-156067, and Japanese Unexamined Utility Model publication No. 59-58736 disclose air intake systems wherein an intake manifold comprises an air inlet tube, an air vessel and a plurality of branched tubes, with the inlet tube being divided into two intake channels. The air vessel is provided with a rotary or slidable valve which functions to divide the inside of the vessel into two chambers communicated respectively with the two intake channels. The valve is adapted to be closed during low speed operation of the engine to separate the inner space of the vessel into two chambers so that, in the intake manifold as a whole, there are formed two separate intake passages; the air being fed to one cylinder group through one intake passage and to the other cylinder group through the other intake passage. This is equivalent to a situation wherein the cross-sectional flow area of the intake manifold is reduced and the effective length of the intake manifold lengthened, whereby the manifold is matched for inertia supercharging at a low engine speed. On the other hand, the rotary or slidable valve is opened in response to the engine operating at a high speed, so that a common unitary space is formed in the air vessel. In this condition, the intake air streams flowing through the two intake channels are first merged together in the common space in the air vessel and the air is then fed therefrom to the cylinders. This results in the effective length of the intake manifold being equivalently shortened so that the manifold is matched for inertia supercharging at a high engine speed.
FIG. 4 of Japanese Unexamined Utility Model publication No. 58-129063 illustrates another air intake system based on a similar principle. In this system, the inside of an air vessel is permanently divided into two volumetric chambers by a longitudinally extending partition wall. One of the volumetric chambers is communicated with one of two groups of engine cylinders through a group of branch tubes, and the other volumetric chamber is connected to another group of cylinders through another group of branch tubes. The partition wall 13 has a port 14, called a balancing passage, which is provided at a location most remote from an air inlet of the vessel; the balancing passage serving to communicate the two volumetric chambers 5A and 5B with each other. A control valve in the form of a poppet valve 11A is provided at the balancing passage 14 and is adapted to be opened at a high engine speed and closed during a low speed operation of the engine. When the poppet valve 11A is closed at a low engine speed, to isolate the volumetric chambers 5A and 5B from each other, intake air is supplied to each cylinder through only one of the volumetric chambers thereby producing the effect of an extended effective length of the intake system. At a high engine speed, the poppet valve is opened to permit the air to flow through the balancing passage from one volumetric chamber to the other, so that the engine cylinders operating on the intake stroke will draw the intake air directly through the corresponding volumetric chamber, on the one hand, and a certain amount of air flowing from the other volumetric chamber through the balancing passage into the corresponding chamber, on the other hand. This will produce the effect of a shortened effective length of the intake system and reduce the flow resistance thereof, thereby improving the output torque at a high engine speed.
However, the problem of this intake system is that the use of a poppet valve necessarily results in a limiting of the dimension of the balancing passage, so that it is difficult during the high speed operation of the engine to ensure an air communication between the two volumetric chambers free enough to permit an adequate amount of air to pass through the balancing passage. This hinders a full enjoyment of the effect of the shortened effective length of the intake system at a high engine speed.
Another disadvantage is that when the poppet valve is opened, it remains situated opposite the balancing passage and in the close vicinity thereof so that the stream of air flowing through the passage is obstructed by the opened valve, thereby increasing the flow resistance through the intake system. A further disadvantage is the difficulty of machining the balance passage due to the passage being located at one of the branch tubes.